The present invention relates to flexures, and more particularly to a high stiffness flexure and a process of making a high stiffness flexure.
A flexure is a flexible mechanical member connecting two bodies. They may be used as a special bearing or hinge to guide the linear motion of one or both of the bodies, such as a stage. A properly designed flexure is extremely stiff in every direction except the direction of motion. A major benefit of a flexure guided stage is the complete lack of friction, since no part is moving against another. Further, there is no backlash in a flexure stage. Most flexure systems are designed to guide motion linearly, although rotary flexures also exist. A flexure system can be constructed to be stiff or compliant in any number of allowed axes. A simple linear flexure is a strip of metal, or other strong material, that is securely attached at one point to one body and securely attached to a second body at another point. In a well designed flexure, the strip is made rigid for most of its length, but is weakened so that it can bend in short segments next to the attachment points.
An inherent limitation of a flexure is its limited motion. Since a flexure is actually a bending beam, its motion is limited by its limited flexural strength. Increased range of motion is allowed by increasing the length of the flexure, but this compromises other qualities of the flexure. Increasing the length of the bending segments also makes the flexure less stiff in the other axes of motion. Increasing the overall length of the flexure increases the mass of the rigid section of the flexure between the two short bending segments. The rigid section between the two bending segments forms a spring-mass system which has resonances. Resonances are undesirable, but higher resonant frequencies are preferable to low resonant frequencies.
The ways that flexures are usually formed can easily lead to a resonant frequency that is low enough to be a real problem or performance limitation in a motion system. Currently, there are two primary methods of creating a flexure. One method, an additive process, shown in FIG. 1A, is to start with a relatively thin strip of flexible material 1 and add rigid strips 2 and 3 in the central section leaving thin bending segments 4 and 5 near each end 6 and 7. Another method, a subtractive process, shown in FIG. 1B, is to start with a relatively thick bar 8 of material and machine notches or slots 9 and 10 near each of the two ends 11 and 12 to form the bending segments with the relatively rigid original bar in between. There are advantages and disadvantages to both approaches. The subtractive process is relatively more expensive, can make a stiffer flexure, and has no joints. The additive process can be less expensive, the bending elements and the central stiffener can be made of different materials, but it has mechanical joints bonding the parts, which can create additional vibration problems. In both cases, the central stiffener is typically solid which makes it stiff but it is also typically massive so that it has a relatively low specific stiffness (i.e., stiffness to mass ratio), which produces a low self-resonant frequency. FIG. 1C shows a flexure which has a center stiffened section created by bending sides 52 and 53 up 90 degrees, forming a “U” channel, leaving segments 50 and 51 to flex. This design does have a higher specific stiffness than a solid bar, but if the sides are made tall to maximize stiffness the sides become cantilevered masses with an additional low self-resonant frequency of their own. With typical flexure design and fabrication practices it is difficult to create a flexure that is compliant enough to act as a flexure and has a high self-resonant frequency.
It generally takes a system of flexure elements to create a functional unit that allows for motion primarily in one axis and is stiff in the other axes. Two such systems are shown in FIG. 2A and FIG. 2B. The system in FIG. 2A allows bodies 15, 16 to move laterally with respect to each other, shown by the arrows. The flexure elements 13, 14 constrain the motion so that the bodies remain parallel. The system in FIG. 2B couples a rigid core 20 with a rigid outer ring 21 with three flexure elements 17, 18, and 19. This system constrains bodies 20, 21 to move concentrically with respect to each other (i.e., in and out of the page). In each case, the flexure elements are formed as described above and shown in FIG. 1A and FIG. 1B. In such systems, each flexure element has resonances due to its shape and material properties (the same if the flexures are the same), and the system has complicated resonances.
What is needed is a flexure with a high stiffness. Furthermore, what is needed is a method for constructing a high stiffness flexure and/or flexure system that is composed of flexure elements with high self-resonant frequencies assembled in a way that minimizes the additional problems of a system. The present invention solves these and other problems by providing high stiffness flexure and method of making a high stiffness flexure as describe below.